* Beagles come in standard size. There's no recognized breed standard for a "mini" beagle.
* Color is not related to size. A beagle can be black and standard size or black and a bit smaller than average. But, color doesn't dictate a dog's size.
If you are looking for a smaller beagle, you might consider a beagle mix, which could have a black coat. However, remember that even with a mix, size and coat color are not guaranteed.